w1_therm: adding ext_power sysfs entry

Adding ext_power sysfs entry (RO). Return the power status of the device:
 - 0: device parasite powered
 - 1: device externally powered
 - xx: xx is kernel error

The power status of each device is check when the device is
discover by the bus master, in 'w1_therm_add_slave(struct w1_slave *)'.
The status is stored in the device structure w1_therm_family_data so
that the driver always knows the power state of each device, which could
be used later to determine the required strong pull up to apply on the
line.

The power status is re evaluate each time the sysfs ext_power read by
a user.

The hardware function 'read_powermode(struct w1_slave *sl)' act just as
per device specifications, sending W1_READ_PSUPPLY command on the bus,
and issue a read time slot, reading only one bit.

A helper function 'bool bus_mutex_lock(struct mutex *lock)' is introduced.
It try to aquire the bus mutex several times (W1_THERM_MAX_TRY), waiting
W1_THERM_RETRY_DELAY between two attempt.

Updating Documentation/ABI/testing/sysfs-driver-w1_therm accordingly.

Signed-off-by: Akira Shimahara <akira215corp@gmail.com>
Link: https://lore.kernel.org/r/20200511203650.410439-1-akira215corp@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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